Trace the face of the stethoscope onto the plastic, and cut out with scissors. File to fit.

Mount the diaphragm to the stethoscope with the rubber ring, then shrink it to fit with the lighter.

Scrounge and make do is the rule when you find yourself in the middle of a major
crisis. I found myself having to MacGyver toys, door latches, satellite dishes,
and in this case, a stethoscope that had lost its diaphragm. You can use a stethoscope
in that condition, but only just barely. So, with my handy Leatherman, a borrowed
cigarette lighter, and the blister pack from some batteries, I improvised a repair.